Why the tiniest fry need living water
Some fry hatch with mouths so small that the usual first foods — even newly hatched brine shrimp and microworms — simply do not fit. Gourami, betta and many rainbowfish fry spend their first days hunting organisms a fraction of a millimetre long, and if nothing that size is in the water, they starve in the middle of apparent plenty. Green water and infusoria fill that gap: one is a soup of single-celled algae, the other a bloom of microscopic animals that feed on bacteria and decaying plant matter. Both are grown in jars on a windowsill rather than the fry tank itself, so you can feed the fry tank by the spoonful, control the density, and keep the culture's mess out of the nursery. Plan to start a week before you expect free-swimming fry; cultures cannot be rushed into existence overnight.
Growing green water
Fill a clean jar or bottle with water from an established tank — it carries the algae species and the trace nutrients they need — and stand it in the brightest light you have: a sunny windowsill or close under a lamp on a long day. A pinch of plant fertiliser or a few drops of the water from a boiled vegetable gives the algae a nutrient push, and a gentle airstone keeps cells suspended and fed with air. Over several days the water passes from clear to hazy to an unmistakable pea-soup green; that deep-green stage is peak density and the moment to start using it. Green water doubles as food and as a feed crop — infusoria and daphnia both thrive in it — so a surplus jar is never wasted. Keep it bright and it holds for days; let it crash and it turns yellow and smells, at which point you discard it and restart from a spoonful of any jar still green.
Starting an infusoria culture
Infusoria need a bacterial bloom to graze on, and the classic way to raise one is plant matter: pour boiling water over a lettuce leaf, a pinch of hay or a small piece of banana skin in a jar, let it cool, then top up with established tank water, which seeds the microscopic life. Stand the jar somewhere warm and lit and wait — the water clouds as bacteria multiply, then clears slightly as the infusoria bloom on them, and a drop under a magnifier or strong light shows specks drifting and darting. That is harvest time, usually within the week. Draw the culture water from the clearer top layer rather than the decaying bottom, and expect a mild sour smell; a culture that smells genuinely foul has gone anaerobic and should be discarded. Like green water, each batch peaks and fades within days, so stagger two jars a few days apart.
Feeding fry and stepping them up
Feed the fry tank little and often: a few spoonfuls of green water or infusoria culture spread across the day, keeping the water faintly hazed with food so a hunting fry bumps into a meal within a few body lengths in any direction. Overdosing is the danger — decaying culture in a fry tank consumes oxygen and fouls fast — so add the next spoonful only when the water has cleared, and siphon the bottom gently with airline tubing as debris collects. Within days the fry outgrow the jar foods; graduate them to microworms, then newly hatched brine shrimp, keeping the infusoria going alongside until every fish in the tank is visibly taking the bigger food. Because every stage overlaps, a written rotation helps: KinsLe's Care Teams can carry the fry tank's culture duties — new jar started, peak jar in use, step-up day noted — so no stage lapses while the fry are at their most fragile.